Pull pin devices are known as a mechanism for locking two bodies together when the pull pin is extended, and for unlocking the two bodies when the pull pin is retracted. In a device of this type, the pull pin is constrained by a housing attached to a first body to move only in the axial direction. This pull pin is often spring-loaded to bias the pin in the extended position, where it extends into a hole or pocket in a second body, thereby positively locating the second body relative to the first body. When the pull pin is retracted from the hole or pocket in the second body, the second body is able to move relative to the first body. Often, the second body will have a plurality of holes or pockets, so that the second body can be positively located in any one of a plurality of set positions relative to the first body when the pull pin is extended, and can be moved between these set positions when the pull pin is retracted.
A typical use for a pull pin assembly is to adjust the height of one body relative to another. These devices are used quite heavily in the fitness industry. For instance, a padded seat used in a weight machine, such as a bicep curl machine, would typically be made adjustable to allow users of different heights to be seated at the correct height to allow them to interact with the weight machine in the proper ergonomic position. A typical seat height adjustment mechanism would have a padded seat attached to a telescopic tube mechanism, where a first, smaller diameter tube would be able to slide up and down inside a second, larger diameter tube. The first, smaller tube would typically have a plurality of holes punched or cut along its axis. The second, larger tube would have a pull pin assembly attached to it and be designed to have the pull pin aligned with the holes in the smaller tube. Whenever the pull pin would be retracted, the first, smaller tube would be able to slide up and down inside the second, larger tube, allowing the padded seat to be raised or lowered to the desired height. To lock the padded seat at a specific height, the pull pin would be extended into one of the plurality of holes in the smaller tube, thereby preventing the smaller tube from moving relative to the larger tube.
However, because this pull pin design requires certain manufacturing tolerances to ensure that all of the moving components can move smoothly with respect to one another (for instance, the pin has to be able to align with each of the plurality of holes; the holes need to be large enough in diameter to always accept the pull pin; the inner tube has to be smaller than the inner diameter of the larger tube to allow the smaller tube to slide within the larger tube, etc.) these tolerances will often add up to allow some motion between the multiple components, even when the pull-pin is engaged in the “locked” position. This relative motion in the nominally “locked” position will often impact the feel of the machine in an undesirable way (machine has unstable, sloppy, loose, or wobbly feel), and could even cause injury to a user in certain circumstances, if the supposedly “locked” mechanism were to shift or wobble at the wrong time. To reduce this undesirable relative motion in the nominally “locked” position often requires the application of very tight manufacturing tolerances, which can greatly increase the cost and complexity of the apparatus. Additionally, tight tolerances can often make the moving components more difficult to move, thereby increasing the difficulty of use.
Tapered pull pins have sometimes been used to remove some of the undesirable motion in the system. By using a pull pin having a tapered end slidingly engaged with a first body, and having a second body with one or more receiving holes that are smaller than the largest diameter of the of the tapered pin, the tapered pin can be inserted into any one of the holes to lock the two components together. The tapered pull pin acts just like a normal pull pin in that it allows the two bodies to move with respect to one another when the pull pin is disengaged, and it locks the two bodies together when the pull pin is engaged with the receiving hole in the second body.
However, the tapered end of the pull pin allows the tapered pull pin to fill up some of the hole clearance, thereby reducing some of the undesirable relative motion between the two bodies. The leading end of the tapered pull pin easily goes into the small receiving hole at first, but as the pull pin moves axially into the receiving hole, the tapered end of the pull pin causes the cross section at the entrance of the receiving hole to increase until it fills the receiving hole. Therefore, using a tapered pull pin can remove the clearance due to differences in the diameter of the receiving hole and the diameter of the tapered pull pin. However, this does not remove all of the undesirable relative motion between the two bodies. The tapered pull pin itself must be tightly constrained by the first body to minimize tilting or rocking of the pull pin, which would allow motion between the first and second bodies. The axis of the tapered pull pin must be tightly constrained to align with the location of the one or more receiving holes, because any misalignment could allow motion between the first and second bodies. The angle of the taper is important too, because a long taper angle will require a very long throw (large amount of axial travel of the pin to fully engage the receiving hole) while a short taper angle can allow the tapered pull pin to back out in the axial direction, allowing even more motion between the first and second bodies. Therefore, while a tapered pull pin can reduce some of the stack-up of tolerances that allow relative motion between the two bodies, it cannot eliminate all of the stack-up of tolerances that allow relative motion between the two bodies.
Clamping mechanisms, such as cam locks, have often been used to either augment or replace pull-pin mechanisms. The clamping mechanism is used to clamp the two bodies together to reduce any relative motion between the two clamped bodies. But these mechanisms are often more expensive, require additional components, and are often more difficult to use. Because clamping forces can be quite high, clamping mechanisms typically have force amplifying components (such as a lever on a cam lock) that allow a user to apply the needed clamping force required to prevent motion between two bodies. However, these force amplifying components also can make it difficult for a user to judge when then have reached the optimum clamping force. Applying too little force can make it appear that two objects are clamped together tightly, but then allow the two bodies to dangerously slip during later use. Applying too much force can cause damage to the components. Additionally, the large clamping forces in turn create large frictional forces, often making it difficult for a user to lock or unlock the clamped components. Again, the addition of these mechanisms can greatly increase the cost and complexity of the apparatus.
There remains a need for a locking and unlocking apparatus which will securely lock two bodies together such that there is relatively little relative motion between the two bodies when the locking mechanism is engaged, while still offering the ease of use, reliability, cost advantages, and reduced complexity of a lower tolerance device.